For example, for a pressure-sensitive adhesive and a pressure-sensitive adhesive tape to be used in respective fields of vehicles such as automobiles, trains, buses and the like, home appliances such as televisions, personal computers, air conditioners and the like, OA facilities such as facsimiles, photocopiers and the like, aircraft, boats and ships, houses, factories and the like, polyvinyl chloride (hereinafter to be referred to as PVC) has been widely used as an adherend or a substrate. However, incineration of PVC may produce toxic gases such as dioxins, chlorine gas and the like. Out of the growing environmental awareness in recent years, there is a move toward limitation on the use of PVC and conversion to materials posing less environmental burden. As a result, an alternative to PVC has been strongly demanded. Similarly, an alternative to a halogen atom-containing pressure-sensitive adhesive, and an alternative to a pressure-sensitive adhesive tape comprising a halogen atom-containing material as a substrate are also desired.
For an alternative to the halogen-containing materials such as PVC and the like, olefin polymers such as polyethylene, polypropylene and the like have been increasingly used from the aspects of cost and flexibility. A pressure-sensitive adhesive tape using these materials as a substrate, and a pressure-sensitive adhesive and a pressure-sensitive adhesive tape for which a product using these materials is an adherend have been considered.
When such olefin polymer is used as a substrate or adherend of the adhesive tape, rubber and acrylic pressure-sensitive adhesives are used from the aspect of cost.
When a rubber pressure-sensitive adhesive is used, however, adhesive strength to a substrate or adherend made of an olefin polymer becomes too strong, which poses problems for a pressure-sensitive adhesive tape, such as unattainable unwinding of a tape, and the like. Upon unwinding, the tape often suffers from stretched substrate, broken substrate and the like. When a rubber pressure-sensitive adhesive is used for an adherend made of an olefin polymer, problems occur due to its excessive adhesive strength, as evidenced by a remainder of the adhesive on an adherend upon peeling off (i.e., adhesive residue), and the like.
When an acrylic pressure-sensitive adhesive is selected, problems occur such as weak adhesive strength to the surface of a substrate or adherend having poor polarity, such as olefin polymer and the like, and the like.
For example, in the case of a binding tape, the unwinding force of the tape and the fastening force thereof are both weak, thus failing to achieve sufficient winding. In addition, since the adhesive strength to the substrate (backside thereof) is weak, the tape end may defectively peel off after winding the tape and the like.
The peeling of the end can be prevented by crosslinking the adhesive to increase cohesive strength and the like, but the adhesive strength simultaneously decreases, and the unwinding force becomes still weaker.
For compensation, addition of a tackifier (e.g., rosin tackifier, terpene tackifier, aliphatic petroleum hydrocarbon (C5) tackifier, aromatic petroleum hydrocarbon (C9) tackifier and a hydrogenated compound thereof, and the like) to an acrylic pressure-sensitive adhesive in an attempt to increase the adhesive strength of the adhesive has been considered.
However, it has been elucidated that, even if such improvement is made, a tape, wherein the substrate is an olefin polymer and the adhesive is an acrylic pressure-sensitive adhesive, tends to show weaker backside adhesive strength after heating and preservation, and weaker adhesive strength after adhesion to an olefin polymer adherend, heating and preservation, peeling off and adhesion again to the olefin polymer adherend, as compared to the initial adhesive strength. In addition, there have been problems of degraded unwinding force and easy occurrence of end peeling.
When the flexibility and stretchability necessary for a pressure-sensitive adhesive tape are considered, moreover, the use of an olefin polymer having comparatively higher flexibility as an olefin polymer, such as ethylene-vinyl acetate copolymer (hereinafter to be referred to as EVA), ethylene-ethyl acrylate copolymer (hereinafter to be referred to as EEA) and the like, has been considered. Particularly when these olefin polymers are used, degradation of the adhesive strength after heating and preservation is significant. It has been found that higher the content of a polar component, such as a vinyl acetate component, an ethyl acrylate component and the like, is, such tendency becomes more noticeable.
The decrease in the adhesive strength after heating and preservation is considered to be attributable to the improved compatibility of the tackifier in the acrylic pressure-sensitive adhesive with an olefin polymer in a substrate or an adherend during heating and preservation, which in turn caused the shift of the tackifier into the substrate or adherend.
In fact, when the amount of the tackifier that shifted into the substrate or adherend was quantified, one having lower adhesive strength showed a greater amount of shifted tackifier.
When the amount of the tackifier increases, the cohesive strength of the adhesive becomes lower, which in turn problematically causes a cohesive failure upon peeling and the like.
Accordingly, it is an object of the present invention to provide an acrylic pressure-sensitive adhesive composition having high adhesive strength (particularly initial adhesive strength) to an adherend made of an olefin polymer material heretofore been considered to be poorly adhesive, wherein degradation of the adhesive strength after heating and preservation has been suppressed; a pressure-sensitive adhesive tape comprising the adhesive composition; particularly a pressure-sensitive adhesive tape comprising an olefin polymer material as a substrate, which shows a sufficient unwinding force when used for binding, and which is free of end peeling after binding; a pressure-sensitive adhesive composition substantially free of a halogen atom; and a pressure-sensitive adhesive tape comprising a pressure-sensitive adhesive and a substrate substantially free of a halogen atom.